In oil insulated inductive devices, such as power transformers, mineral oil is typically used as an insulating fluid between inner parts subject to different electric potentials. The inner parts of an inductive device normally comprise a magnetic core, windings, and an electrical insulation system which provides insulation between parts having different electric potential. In particular, in the main duct of an inductive device a certain distance in oil should be kept to avoid dielectric breakdown during tests and service.
One typical solution of the insulation between windings in the main duct for core type designs implies the use of cylindrical barriers made of e.g. pressboard to divide oil spaces in the radial direction. This subdivision greatly improves the dielectric strength for the whole width of the main duct and it allows in practice to reduce its width significantly. The pressboard barriers are normally cylindrical and they are placed concentrically between the inner and outer winding in the main duct during the manufacturing of the inductive device. In order to support the barriers a set of longitudinal bars made of e.g. pressboard are placed evenly around the inner winding or the subsequent inner barriers.
The turns or discs in a winding can be arranged so that they are separated by pressboard spacers in the axial direction. These spacers provide space for electrical insulation as well as the flow of cooling oil. As they are placed evenly around the circumference of the winding, they are set in their positions by coupling to a corresponding longitudinal bar.
It has been identified that the oil regions delimited by winding conductor, winding spacer and longitudinal bar are heavily stressed under voltage conditions during tests and operation of an inductive device. In particular, during a lightning impulse stress, in these regions so called oil wedges can provide a point of initiation of an electrical flashover. In order for the flashover to be developed, a path for propagation must be formed and it must be connected to a surface of different potential. A streamer can propagate from the oil wedge across the oil space close to the wedge in the duct closest to the winding. A streamer can also propagate along the surface of the longitudinal bar until it reaches the cylindrical barrier and continue from that point along the barrier itself.
One example of an inductive device which has an insulation system that reduces the risk of flashovers is disclosed in GB191513586. The electrical transformer disclosed therein has windings composed of slab-like units, each made of insulated spirally wound flat wire. These units are separated by spacers which are interlocked at their ends with longitudinal spacer bars.
Existing electrical insulation systems do however not provided an adequate protection from streamers propagating from a spacer towards a cylindrical barrier.